We will study the mechanisms by which visual information is transformed between the lateral geniculate nucleus of the thalamus (LGN) and layer 4 of primary visual cortex in cat. The functional transformations between receptive fields in the LGN and cortex are well known, but little is known of the mechanisms by which these transformations are achieved. We will study the connectivity between pairs of single LGN and cortical neurons and relate the degree of connectivity and its type (excitatory or inhibitory) to the visual functions of these pairs of neurons. Synaptic connections can be inferred by a lawful relation between the firing of the LGN neurons followed, on a short time scale (several msec), by the firing of the cortical neuron. The visual function of these neurons will be assessed with a method of automated receptive-field mapping (spatiotemporal white-noise analysis). This two-pronged study of both connectivity and function will allow us to test specific models of information processing by cortical cells. We will study the cortical mechanisms responsible for the selectivity for orientation and direction of motion in simple cells. The interpretation of our results concerning the connectivity between pairs of LGN and cortical neurons requires a second piece of information: what are the range of LGN afferents available locally for processing by single postsynaptic cortical cells? In additional experiments, we will therefore study the full range of thalamic inputs to any given point in the cortex. The number of afferents recorded and the density with which they are sampled will be far greater than in the more difficult LGN- cortex experiment. Thalamic afferents will be recorded directly in the cortex by silencing the cortical cells pharmacologically. In addition to multi-unit recording and automated receptive field mapping, this study will make integral use of optical imaging, a powerful new technique for mapping the function of neurons across the cortical surface in vivo. The type and scatter of afferents recorded in the cortex will be correlated with the local receptive field (e.g. their ocular dominance, preferred orientation, or color specificity) measured with imaging. This basic research on information processing in the visual cortex should further our knowledge of general mechanisms of cortical function.